Hi!
During my IR training with a French ATO last year, I was taught a technique they called “variation” (in french). It is used to change radial when flying inbound any kind of fix (VOR, NDB…), by initially diverging 30 deg from the initial radial, and then converging by 10 deg each time the RMI shows the relative bearing has reached 40 deg.
I’ve been wondering since then… is this technique taught outside France?
The typical scenario was re-positioning the aircraft to reach the IAF along the track of the Initial Approach leg. A typical case is shown here below:
We were flying the NG4 arrival on radial 084° outbound NG/inbound CM, and the intention was to successively fly the ILS Y (IAF at CM, initial leg 004° outbound).
A few minutes before reaching CM, the instructor told me to use the “variation” technique to change from 084° to 004° inbound CM, because “you cannot arrive at the IAF with an angle greater than 30° from the following outbound course”. So, two alternatives: either you enter the hold, or you use the “variation”.
I have done some search on the internet, without being able to find any reference to this technique. Additional, I haven’t found anything stating that “you cannot arrive at the IAF with an angle greater than 30° etc.”. Maybe PANS-OPS?
Any thoughts? And, how would you negotiate this with the ATC? Which phraseology?
Do I understand correctly that
a bit like this?
(nor real precision in the sketch)
Anyhow, here is my view
I think the instructor probably meant to say that when you approach a turn at more than a 30 degree angle, you need to anticipate the turn somehow, otherwise you will overshoot too much.
I would normally just anticipate the turn, and as a general technique with an NDB, it is not a bad idea in general to err on the side of turning a bit early to then intercept the outbound track than leaving it to late and overshooting, which is more effort to correct.
And if I interpret what he taught you correctly, this is a technique to arrive at a particular track over a beacon. A useful skill to have (basically, a smooth intercept of the outbound track on the inbound side of the beacon before reaching it), but not sure I would use it in these circumstances.
It’s one of several techniques taught in France for changing VOR and ADF radials as well as DME arcs. Since I did it I can’t remember ever having had to use it in real life especially with RNAV approaches becoming more prevalent.
Probably useful if you don’t have any indication telling you how far away from the beacon you are, and superfluous if you have a DME or RNAV to allow you to anticipate the turn.
Other than that, the good old rule of thumb – start the turn around 30 seconds before reaching the beacon for a 90 degree turn – works nicely. Or in general – turn about as much early as it takes to fly the turn, so 30 seconds for a 90 degree turn, 15 seconds for a 45 degree turn.
This is bit early so you you will need to hold off the final bit of the turn until you actually intercept the outbound, but it works a treat in practice.
Giovand wrote:
Additional, I haven’t found anything stating that “you cannot arrive at the IAF with an angle greater than 30° etc.”. Maybe PANS-OPS?
Yes, PANS-OPS. That should be taught in the IR ground school. To be precise, the rule says that you must approach the fix where the base turn starts at a course of ±30° of the outbound course, except that if the outbound course differs more than 30° from the reciprocal of the inbound course, then you can extend the 30° on that side up to the reciprocal of the inbound course.
So in this case, if the outbound course was 020°, then you could approach CM on any course between 348° and 050°.
Cobalt wrote:
I think the instructor probably meant to say that when you approach a turn at more than a 30 degree angle, you need to anticipate the turn somehow, otherwise you will overshoot too much.
That’s another way of putting it — but it’s not “turn anticipation” in the sense of RNAV.
Cobalt and Gallois, thank you both for your replies!
Yes, Cobalt, your understanding is completely correct. They also called this technique “30/40”, which would become “40/50” if the wind was pushing you toward the fix.
I’ve always been uncomfortable with it, mainly because you’re not 100% sure of being clear of obstacle/terrain (as a minimum, you have to check MSA and Minimum Vectoring Altitudes before starting the maneuver), and also because – if this maneuver is not a standardized one – I do not know how to make ATC exactly aware about what I am gonna do, and which portion of the airspace I need for it: what if there’s another traffic converging inbound the CM from the South, same altitude? Even if, in this case, I guess, their answer would be “Negative, Sir”…
Maybe accepting the fact that on a good tracking without distance indication you will surely overshoot, hence a better strategy is to get sense of the distance by going off-track and then correcting (2 moves of shallow turns) instead of overshooting off track and correcting (3 moves with steep turns)?
For accuracy, depends if doing this at about 1DME? 5DME? or 10DME?
Good suggestion Ibra, thanks!
For information, the above 30/40 maneuver is supposed to begin at 3 min from the station
@Giovand flight instruction, particularly IR is different in France from say the UK.
In France they rely much more on mental calculations for instance whereas in the UK they are much more used to whizzy wheels and computers. However some mental calculations, the UK way are much easier to get to grips with, eg mentally calculating true airspeed and altitude, the 1 in 6 rule etc.
Also getting used to the TOP the CAP the OBS the Radio instead of the 5Ts time turn twist throttle talk is not natural to a non French national but once you’ve got used to it it is difficult to go back.
I think I can perhaps narrow down which of the schools you are at. If I am right, they are excellent for the would be professional pilot, so if you can bear with it, do.It will serve you well in the long run and if you don’t understand something make the instructor go through it slowly on the ground. Once you get it its like a light bulb going on.
Good luck.
